SharePoint Designer has retired. Here is the part that affects your forms.

Split the problem in two

The two halves go to different places, and no single product replaces both.

The workflow half goes to Power Automate. That is Microsoft's replacement and the migration tooling converts a subset of common actions for you. Forms 365 does not replace a workflow engine, and we would rather say so plainly than let you find out later.

The forms half is the part with no automatic migration and no like-for-like replacement. Microsoft points to Power Apps and Microsoft Forms, neither of which does what InfoPath did for list forms. Either way those forms have to be rebuilt in a supported designer. That is the part we do.

What to do, in order

1. Inventory what actually broke. Separate the list of dead workflows from the list of dead forms. They will not be the same list, and some items appear on both.
2. Rebuild the busiest form first. Point a supported designer at the same SharePoint list, recreate the fields and rules, and publish. Your list data is untouched throughout.
3. Wire the workflow back up separately with a Power Automate flow triggered on that list. Because the form writes real list items, a list trigger picks them up.
4. Then work down the rest in order of how much each form is actually used, rather than the order you found them in.

Your data is fine

Losing a form or a workflow does not touch the SharePoint list items behind them. This causes real panic and it should not. You are rebuilding how people enter and route the data, not recovering the data itself.