Alternative to Latte's buttons, app menu & title widgets compatible with Plasma 6?

In Plasma 6, those widgets appear to no longer work in Plasma 6.

Are there alternatives to those ones?

Keep an eye on the KDE Store.
For Plasma 6 there already is Window Title Applet 6 - KDE Store for example.

It will certainly take some time until things are rewritten for Plasma 6 (especially from third parties).