>> >  2. a regression of r-rgdal introduced by your commit
>> > f9d328833fc1f5d0fb76b61b12d1a3cb013932e6
>>
>> Replacing proj.4 with proj in the r-rgdal package seems to fix this
>> regression. Can you confirm?
>
> Maybe, but it is not what the user expects. Upstream explicitly
> mentions proj.4, see [1].

> [1] https://cloud.r-project.org/web/packages/rgdal/index.html

No, upstream says that both proj (aka proj6) and proj.4 are
supported. Quoting [1],

"From 'rgdal' 1.4.1, provision is made for 'PROJ6' accommodation, ..."

> The question is: why proj instead of proj.4 in libgeotiff?
> The bug [1] cannot be solved using proj.4, why?

proj and proj.4 are different versions of the same software, with proj
being the newer version. See
https://proj.org/faq.html#what-happend-to-proj-4 . I say we completely
deprecate our proj.4 package and replace all occurrences of proj.4 with
proj.

WDYT?