You obviously did not read the whole discussions in the threads I linked to in post #4 above. For example Themes moved under "Obsolete Features" - #33 by lubos states in more depth (just part of post replicated here):
So I’m simply observing where the technology is heading, internal PDF generator which was causing me a lot of headaches has been made obsolete and the only reason why custom themes are using liquid markup is therefore no longer justified.
and
Customization of invoices will be through javascript which is easier to understand and debug than liquid markup. I mean anybody who knows how to make websites will be able to customize the look of invoices (or other documents) without knowing anything about Manager. That means there will be more talent available for businesses to choose from to implement customizations.
and
I’m not yet there. This is multi-step process to get there but once I get there, all users will naturally prefer javascript over liquid markup for their customization needs. Liquid markup is unfortunately all or nothing proposition - you have to have the entire invoice rewritten even for the tiniest customizations. Javascript allows progressive enhancement.
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However, having said all that! The implementation of the alternative is well overdue. The announcement to make themes obsolete was made by @Lubos on Aug 9, 2022 10:22 PM and he stated in that announcement at Themes moved under "Obsolete Features" that:
But if you are not using themes at the moment, you should not start using them from now on.
It is now 16 March 2024 more than 1.5 years after @Lubos advised this and despite many users explaining that they need customization of their themes nothing happened. The post at Themes moved under "Obsolete Features" - #42 by lubos in response to @MarV on Aug 29, 2022 10:47 AM, about 20 days later said [emphasis added]:
But it has not been done, only some work on footers, and then in the knowledge that the obsolete themes would still work until whatever solution would appear left us waiting, while those that followed the advise on 9 August 2022 “…you should not start using them from now on…” are still only able to post a logo and some sort of footer to their invoices because they obeyed!