The STK was deprecated some time ago but we still rely on it. We managed to update up to last 5.7 version but starting from 6.1 the templates are not working anymore.
There is not clear when you read the documentation whether STK is still usable with magnolia 6 or not.
Actually, here: https://documentation.magnolia-cms.com/display/DOCS57/How+to+create+a+custom+webapp+with+STK+modules it even says it should not work anymore with 5.7.
Is there a clear statement about the STK working with newer Magnolia versions?
No, it is not possible.
I have asked magnolia directly and they confirm that STK is not expected to work in version 6.
https://jira.magnolia-cms.com/browse/SUPPORT-16220
The documentation is a bit confusing and for some reason they do not clearly say from which version STK won't work anymore.
We were able to make it work up to version 6.1.8 but it is not worth to keep it working with workarounds as any magnolia or module update could brake again.