<Updated the whole description, Clarified some things, and rewrote my report>
I noticed that the character limit for book titles in 1.20.2 is 15 though they were 16 previously, after some investigation I found this bug going back to 20w17a (1.16 Snapshot) with no mentions in the changelog. Hence this was obviously not intended I kept this report up. To sort out some confusion, I had a typo in my first submission that misspelled 1.20.1 to 1.12.1 which raised obvious confusion, I hope this resolves that.
Furthermore if this issue is recognized and tackled I suggest the limit being raised, as it makes no sense that I can rename the book in an anvil with 50 characters, but cannot sign it with a 50 character title, also copying/pasting text and moving the cursor in the menu would create some parity to writing in the book itself, editing the title/item name in an anvil, and recent changes to signs.
I want to apologize if I was a bit rude in the comments, I did not intend to offend anyone for being confused about my typo.
Let's get this behind us and make this game better by removing 1 bug at a time 🙂
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After 2 hours of investigation and loading 21 Different Versions of the Game I found that this bug first occurred in 20w17a, a 1.16 Snapshot, as seen in the attached Screenshots. This confirms, that this is indeed a bug as this is nowhere mentioned in the changelog.
Now that that is off the table, I would be happy if it could be recognized as what it is, thank you.
(Both worlds were freshly created, creative, cheats enabled, no experimental features)

Thank you for your report!
We're actually already tracking this issue in MC-210318, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote or any additional information to the main ticket it would be appreciated.
Please be sure to use the search feature before creating a ticket to see if the issue has already been reported.
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Thank you for confirmation,I did search using the internal search engine and did not find anything, hence I made my report. Funnily enough my report is now ranked above the original report on google search, where I could find it. I will add my information to the old report, thank you
All those wikis (including the "official" wiki) are community maintained, none of them are actual sources of intent. Unless it recently changed without a mention in the changelogs, this is a mistake on those wikis, and not a bug.