
Per my test, "Enable this form to be filled out by using a browser" option is supported in SharePoint online classic library experience.
(Note, I migrate it via re-publishing existing form to SharePoint online site in infopath. )
InfoPath Forms Services requires SharePoint Plan 2, Office 365 Enterprise E3, or Office 365 Enterprise E5.
However, in SharePoint online, Microsoft Power Apps is the recommended solution for creating and delivering custom forms for SharePoint lists.
We suggest you replace your InfoPath forms with one of the following alternatives:
- Use Microsoft Power Automate and Microsoft PowerApps.
- Move complex behaviors to the new add-in model and client-side developments.
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