Honestly, assembly language bitrots far faster than other programming languages. In my lifetime, the only thing that really comes close to qualifying as "compiler language syntax/compiler planned obsolescence" is Python 2 to Python 3. In contrast, with x86 alone, there's three separate generations of assembly language to go through in that same timeframe.
Python still doesn't do very well on backwards compatibility & bitrot, even after 3. They're constantly deprecating and removing things.
https://docs.python.org/3.12/deprecations/index.html
This obviously improves Python, but also it means you absolutely shouldn't choose Python if you are looking for a low maintenance language.
Cycles of the planned obsolescence here is long, 5/10 years.
Shorter for c++ and similar, than C.
Look, I work on compilers, and I have no idea what you're even trying to refer with "planned obsolescence" here.
And 5/10 years is a very short time in compiler development planning! Prototypeless-functions in C were deprecated for longer than some committee members were alive before they removed from the standard, and they will remain supported in C compilers probably for longer than I myself will be alive.
feature creeps.