And here is the compile-log I got after installing the package on Emacs
27.1:

--8<---------------cut here---------------start------------->8---
Compiling file /home/philip/.config/emacs/elpa/package-1.1.1/package.el at Fri Apr  7 11:58:06 2023
Entering directory ‘/home/philip/.config/emacs/elpa/package-1.1.1/’

In package-tar-file-info:
package.el:1243:8:Warning: ‘(filename (tar-header-name (car tar-parse-info)))’
    is a malformed function
package.el:1247:31:Warning: reference to free variable ‘loop’
package.el:1249:58:Warning: reference to free variable ‘filename’

In package-menu-toggle-hiding:
package.el:3208:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-hide-package:
package.el:3556:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-describe-package:
package.el:3580:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-mark-delete:
package.el:3590:4:Warning: malformed interactive spec: (interactive "p"
    package-menu-mode)

In package-menu-mark-install:
package.el:3599:4:Warning: malformed interactive spec: (interactive "p"
    package-menu-mode)

In package-menu-mark-unmark:
package.el:3607:4:Warning: malformed interactive spec: (interactive "p"
    package-menu-mode)

In package-menu-backup-unmark:
package.el:3613:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-mark-obsolete-for-deletion:
package.el:3620:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-quick-help:
package.el:3652:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-mark-upgrades:
package.el:3745:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-execute:
package.el:3870:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-filter-by-archive:
package.el:4127:4:Warning: malformed interactive spec: (interactive (list
    (completing-read-multiple "Filter by archive (comma separated): " (mapcar
    #'car package-archives))) package-menu-mode)

In package-menu-filter-by-description:
package.el:4149:4:Warning: malformed interactive spec: (interactive (list
    (read-regexp "Filter by description (regexp)")) package-menu-mode)

In package-menu-filter-by-keyword:
package.el:4169:4:Warning: malformed interactive spec: (interactive (list
    (completing-read-multiple "Keywords (comma separated): "
    (package-all-keywords))) package-menu-mode)

In package-menu-filter-by-name-or-description:
package.el:4194:4:Warning: malformed interactive spec: (interactive (list
    (read-regexp "Filter by name or description (regexp)")) package-menu-mode)

In package-menu-filter-by-name:
package.el:4214:4:Warning: malformed interactive spec: (interactive (list
    (read-regexp "Filter by name (regexp)")) package-menu-mode)

In package-menu-filter-by-status:
package.el:4234:4:Warning: malformed interactive spec: (interactive (list
    (completing-read "Filter by status: " '("avail-obso" "available"
    "built-in" "dependency" "disabled" "external" "held" "incompat"
    "installed" "source" "new" "unsigned"))) package-menu-mode)

In package-menu-filter-by-version:
package.el:4273:4:Warning: malformed interactive spec: (interactive (let
    ((choice (intern (char-to-string (read-char-choice "Filter by version?
    [Type =, <, > or q] " '(60 62 61 113)))))) (if (eq choice 'q) '(quit nil)
    (list (read-from-minibuffer (concat "Filter by version (" (cond ((eq
    choice '=) "= equal to") ((eq choice '<) "< less than") ((eq choice '>) ">
    greater than") (t nil)) "): ")) choice))) package-menu-mode)

In package-menu-filter-marked:
package.el:4307:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-filter-upgradable:
package.el:4335:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-menu-clear-filter:
package.el:4344:4:Warning: malformed interactive spec: (interactive nil
    package-menu-mode)

In package-browse-url:
package.el:4528:4:Warning: malformed interactive spec: (interactive (list
    (package--query-desc) current-prefix-arg) package-menu-mode)

In package-report-bug:
package.el:4571:4:Warning: malformed interactive spec: (interactive (list
    (package--query-desc package-alist)) package-menu-mode)
package.el:4571:43:Warning: ‘(reporter-prompt-for-summary-p t)’ is a malformed
    function

In end of data:
package.el:4602:1:Warning: the following functions are not known to be defined:
    file-name-concat, length>, loaddefs-generate,
    native-compile-async, named-let, loop, length=,
    comp-clean-up-stale-eln, format-prompt, substitute-quotes,
    make-separator-line, defvar-keymap, ensure-list,
    macroexp-file-name, dlet
--8<---------------cut here---------------end--------------->8---

The most common issue is that the interactive spec has to be revised to
use only one argument.  IIUC this can be replaced by a `declare' form
with a `modes' entry?

This also gives us a better picture of the unknown functions:

- loaddefs-generate (not provided by Compat)
- substitute-quotes (provided by Compat)
- defvar-keymap (provided by Compat)

I will take a look at what is necessary to back-port `loaddefs-generate'
via Compat.

-- 
Philip Kaludercic