Searching by Memo string fields is still not supported on the forms. I understand the performance concern and that in some cases it can degrade the performance of the whole environment. But some tables still have memo fields and there are many use cases where filtering by them is required and it works fast. On environments with SQL access I often filter by them in SQL queries and I never had problems.

Filtering by memo fields would definitely be an added value and you can add a warning or setup option to cover performance concerns.