IMO, this comes down to a policy decision:
Do we have the resources to support older versions of SME (without hurting dev and new version support)?
If so, is there a real benefit to Lycoris and/or the community to do so?
Historically, users have been urged to upgrade relatively quickly (with a grace period, of course). Perhaps a CentOS/RHEL situation will change the equation.
BTW SME 6 final has been out for almost a year, and in beta3 several months before that. Gordon wrote that in devinfo at a point when everyone had had 6 - 8 months to schedule a minimum impact upgrade to 6.0 final.
This being said, I don't think it would hurt anything to create one < 6.0 support forum, and see if there are enough interested people to keep it active as a "community, not official" support resource.