all the defcustomʼs in eglot.el lack a :version tag. Is that
intentional?
No really, probably oversight and the fact that Eglot being a
GNU ELPA package (presumably easy to upgrade) doesn't usually mean
such version tags are so important.
That and the fact that Eglot doesn't really have that many defcustoms.
So, if you want to add :version tags, go ahead. Note that Eglot
does GNU ELPA double-duty as a :core package, so not sure what
you should put there, but others are perhaps better informed than
me.
João