TYPO3 Core

A bit off topic: where did you get that config.baseURL is deprecated?

Looking at 11.5 docs, I don't see any deprecation being mentioned: https://docs.typo3.org/m/typo3/reference-typoscript/11.5/en-us/Setup/Config/Index.html#baseurl

That's right, "deprecated" is the wrong wording.
But with the possibility of site management since TYPO3 9 there's normally no need for config.baseURL anymore.

My frontend was OK without config.baseURL when the site was running TYPO3 10.4. But after the upgrade to TYPO3 11.5 it crashed and I regard setting config.baseURL as workaround.
Of course, you could also add preceding slashes to page.includeCSS and page.includeJS in order to solve the problem.
But on the other hand I don't see a requirement for that in the docs.

So, all in all I regard this as a bug or at least missing documentation.

A bit off topic: where did you get that config.baseURL is deprecated?

deprecated in TYPO3 Version 12.1

so the question for me what to do: the effect ist still in TYPO3 11.5.26: Add a Slash at the beginning of the files or is there another solution?

a possible Solution via site:base from siteconfiguration

page = PAGE
page.headTag.append = TEXT
page.headTag.append.dataWrap = <base href="{site:base}">

Knut E. wrote in #note-6:

so the question for me what to do: the effect ist still in TYPO3 11.5.26: Add a Slash at the beginning of the files or is there another solution?

Is there a reason not to write directly:

page.includeJS.myjs = /fileadmin/myjs.js

?

Is there a reason not to write directly:

[...]

?

I think both solutions work. The leading slash you have to add to every file. If there are extensions or more files it can be a little bit faster with the other workaround. I don't know if a solution is preferable.