I think ageism will play a part in the selection process, but not always because of the age directly, rather the older staff may have larger pay, pension and benefits costs.
While some companies are reducing their workforce others are growing, some of this is because market share is switching. Traditional business losing to the cloud for example.
There is more software work today than at any point in history. This is only going one way. Until AI will be able to develop software at least.
With respect to age while there are pockets of ageism I think most of the big players and some of the smaller don't have a problem with anyone's age. There are still a lot of companies who can't find enough people regardless of age and I don't think there's a huge salary differential between someone really good in their late 20's to 30's and someone really good who is older. So there's really no economic incentive for ageism. Yes, you can hire someone out of school for cheaper but you need to invest a lot more in them and then they disappear for a more lucrative job...
Big firms are greedy and dumb. IT is a cost centre too them, every tech employee is a moocher sucking cash that could be used in yet another stock buy back to try and keep the stock price high a bit longer so the board can cash out and run.