Typically the front end needed to get a success message to say user created, or user creation rejected (e.g. backend checking on valid postcode, valid username, etc - rejections which would come from two separate services.)
I have also not used a 2pc transaction coordinator in well over a decade.
Also you use the term "saga" which is correct in that what people are often doing here is not in fact a saga (which actually has a pattern description) it is usually a process manager which is a different pattern.
All of this was for no real purpose, and as I said the proof was in the pudding - tens of millions of dollars wasted, hundreds of staff hired and then fired. The domain itself was not complex (e-commerce) but the implementation was ludicrous.
There were basic delivery problems in the project as there was a complete lack of keeping things simple, and massive overengineering.