He hasn't regressed. The Philly team has regressed.
They had Butler, JJ Reddick, TJ McConnell and Shamet - they replaced them with nobody. Lost them all to FA's, doubled down on Tobias Harris and gave him a max (he is not a max player). I think Shamet may have been traded, but either way he was a huge loss. They went and signed Al Horford and Dwight Howard who for the life of me I don't know what benefit you were going to get playing Embiid and Horford starting front court with Simmons at your PG - that line up was funky af and was never going to work. These changes around Simmons impacted his game, he did not regress, the team regressed. What he wasn't doing in 2020, he wasn't doing in 2018, but it's more glorified because players he had doing what he didn't do were gone.
Giddey is playing on a trash team in a small market getting empty stats. He will continue to put up numbers. Compared to Simmons who took his Philly team to the Conference Semi's and they finished with a .624 record AND he was rookie of the year.
You need to stop caring so much about shooting. Not all players need to be great shooters. However, Giddey is a sub-par shooter anyway. He is shooting 39.4% from the field and 25% from 3. Also his FT% is 63% which is as bad as Simmons. He is not a good shooter.