I suspect that accumulate isn't the only algorithm that didn't make it.
Maybe now there's a better way to perform accumulation (folding) over a range and therefore the accumulate is obsolete?
No, accumulate is a perfectly reasonable algorithm, and it's not made obsolete by any other algorithm. The reason for it not being included in C++20 is simply a matter of time. It was considered better to add as much as possible with regards to ranges, without worrying about adding everything at once. Otherwise, there was a risk that none of the constrained algorithms would have made it to C++20, which would have been a shame.
There are still a few algorithms that haven't been constrained yet, as well as the entirety of the <numeric>, and <memory> headers.
Fortunately, there is a proposal to add these remaining algorithms (and I'm optimistic that these will be added in C++23). In fact, the introduction to this proposal answers your question nicely:
“Every time someone asks why we didn’t cover
<numeric>and<memory>algorithms: We thought 187 pages of TS was enough.” — Casey Carter