For version 2 and below Mui (an its depenedcies) were specifcied as a dependancy.

For version 3 and above, Mui is a peer dependency, so you will may to install it yourself. Version 3 and above uses @mui/x-date-pickers Version 7.

date-fns has been upgrade to version 3, so ensure that you are importing the correct AdapterDateFns for your LocalizationProvider. See MUI getting started for more

import { AdapterDateFns } from '@mui/x-date-pickers/AdapterDateFnsV3';

If you are migrating, be sure to read the MUI migration docs to v6 end the migration docs to v7