@Nasser solution is not to do what you describe. Start with an empty preamble and just include packages you need.

@DavidCarlisle but this does not work for me in practice. suppose I added a macro I want to use now everywhere, or added a new package I found useful (fancy header for example)or added a new option to a package I am already using, and want to rebuild my 100,000 latex files with these changes. Now I make the changes in one file only. With your method it means I have to edit 100,000 latex files one by one and copy/add the changes? How could this be better?

Your method is ideal when the tree is very small and has few files, yes. But for a very large source tree, I do not see how to manage what you are suggesting.

but as far as I can tell your 100000 files are really the same document just instansiated with different CAS expressions so yes it makes sense to keep them the same. But even then I'd be suspicious of loading more than dozen packages, do you really use more than that?

@DavidCarlisle as I mentioned, over the years I added/edited this core latex file each time I needed to make a change or fix a problem in some file. I am sure not all the packages are needed for each latex file I have. my whole web site is build using Latex/make4ht. So the number of files is huge, they all include this one core file. if I have to rebuild the whole tree now, it will take 6 months compile time and will break in many places I am sure.

When I have free time, I need to sit down and clean this mess.

@Nasser sure but your setup is massively atypical and if you have ended up with an over-complicated build setup, that's as it is, and not really a reflection of issues with latex as normally used

@DavidCarlisle I easily actively use significantly more than a dozen packages in many of the documents I produce, especially for teaching. That's not as extreme as @Nasser and I don't know how common or otherwise it is. (I'm thinking here of the total number of packages loaded and not just the number loaded in the preamble.)

@cfr <3

To @DavidCarlisle, @JosephWright, @cfr, @AlanMunn, and anyone else in (or from) the regions where it's recognized, Happy Boxing Day.

@barbarabeeton :)

@barbarabeeton :)

@samcarter I came here to ask something and noticed the bounty. I don't even recall last time I received one. Thanks! :-)

@gusbrs Such a comprehensive answer clearly deserves a bounty :) (the system does not allow me to award it yet, need to wait until tomorrow)

Btw, let me ask @daleif something. Do you know if the current state of things for end notes in memoir have been ported to pagenote, particularly hyperref support?

@samcarter The job was done, I had most of it in my own notes. And I certainly have written harder answers which got some meager ticks. It's appreciated. :-)

But I came for a general question. For a package requiring a recent kernel, after UFT-8 being default input encoding, is there good reason to keep localization settings still in LICR?

@DavidCarlisle That's what I hoped to hear. :-) It so much better to keep it in UTF-8. Thanks!

@DavidCarlisle Can I blame you if someone complains?

@gusbrs note though a new format is not enough, you need the user not to have used \usepackage[latin1]{inputenc}

@gusbrs don't be silly. Of course not. @UlrikeFischer is there for you

@DavidCarlisle That was actually my doubt. Should I expect many people doing so? Why would someone do it in a recent document?

@gusbrs old habits....

@gusbrs you could save the inputencoding use \inputencoding{utf8} then set it back at the end

@DavidCarlisle Ok, I see. But, aside from that, is there a use case where someone might actually need a different encoding? If not, I can state the package requires UTF-8 encoding and be done with it.

@DavidCarlisle That's interesting.

@DavidCarlisle I though so. ;-)

@gusbrs no they should never need that, they may have some weird macros doing \defé{...} assuming é is a single byte but better to fix the old macros than stick to a legacy encoding

@DavidCarlisle Ok, I think this is the way. I'll document the requirement, and keep things moving forward. Thanks!

@barbarabeeton Diolch!

@barbarabeeton No Boxes this day, but thanks and happy Boxing Day to you.

@cfr -- you're most entirely welcome.

@egreg -- We don't actually celebrate Boxing Day here. But the temperature is climbing above freezing, and that's something to be happy about. Anyhow, thank you for the good wishes.

@barbarabeeton Be careful what you wish for. Expected max in my city today is 41 °C :(. Air conditioner struggling.