This is pure anecdata, but I don't think I know a single software developer who was unable to continue in the industry past 35 as commonly claimed.
On the other hand, I know tons of former financial employees who became unemployable in the industry at 30 or even earlier.
Ageism may be alive in startups, but in technology as a whole, it doesn't really seem to be a big deal if you are up to date on your skills.
I did the startup thing a bit, it was fun, but I'm mostly over it. I have other things in life I want to focus my time and energy on.
I don't see at all this supposed ageism in software, is this skew coming from startup people? Because in startups, yeah, I see mostly youngsters but that's due to the work environment and pay, I refuse to work in startups for the foreseeable future as I did when I was 20-something. The pay is shit, the hours can be grueling, expectations and stress are way way too high for my current lifestyle. Of course, it can be your golden ticket but it's such a major risk that I don't feel I would like to waste 5 years on it.
Tech industry is ageist. There's a reason why senior+ programmers find it hard to recover their career when detached from the BigTech company they have been long working at. Doubly so, if they work at a company that stack-ranks them at the bottom when they already are on the wrong-side of the 30.