I have yet to see anything official, but for what it's worth, I posted the following response to this question about a month ago, and neither Richard nor anyone else from Sage replied with anything to the contrary:

Richard Jang announced in the Sept. 4 "What's New in Sage 300 version 2020" webinar that payroll will be supported in Sage 300 version 2017 until March 2020.

I would not be at all surprised if one were to call Sage 300 tech. support to hear a stock reply that version 2017 is no longer supported.  However, it seems to me that someone who has been in Accpac product development for as long as Richard probably has some insight.

Ok thank you. So then come March 2020, anybody on Sage 300 2017 loses their payroll because it is subscription? But what if they have paid but just not upgraded?

They won't lose their payroll, they just won't get any further payroll tax table updates. Which means that payroll calculations may be incorrect the next time regulations are changed.

Hmm ... now for Sage 300 2020, the payroll version is still 7.3 so I think if they provide a tax table it should still work past March 2020 unless they change the payroll version.