The postmarketOS Tweaks application has been integrated into and replaced by Phosh Mobile Settings with release 0.52.0. Some of the features have not been carried over or changed due to them being obsolete, flawed, or otherwise made redundant. See the relevant section of the documentation for more details. Note that while Phosh Mobile Settings has "Phosh" in the name, it should work work with any user interface, including for instance Sxmo or Plasma Mobile, just like postmarketOS Tweaks did.
Migration should happen automatically. While all omitted features were excluded with care and consideration, it is possible that there are some use-cases not known to us which nonetheless still would justify including them. If there is a feature which you would like to see brought back, please open an issue in the Phosh Mobile Settings issue tracker detailing what you're missing and why you want it brought back.
Related: !7678