What is the supported infopath version and bit (32 bit or 64 bit) for sharepoint 2016 and custom list forms customized in SharePoint 2016 will be work in SharePoint 2019?
At least, it have to.
As Microsoft states:
Microsoft will ensure that the InfoPath 2013 client will work with SharePoint Server 2019 for the remainder of the client support lifecycle (2026).
InfoPath Services will remain supported, but deprecated, for the SharePoint Server 2019 release.
Personally, I've used Infopath Designer 2013 x64 with SharePoint Server 2016 with no problem.
Also take a look at this answer.