Generally (as you've noted) SOLIDWORKS tends to "support" three major releases back, however "support" is subjective.
Once the final service pack is released (SP5.0) rarely (I know of one or maybe two times) is another release provided to fix something.
Meaning, if you are currently on 2016, it's only "supported" in that you can call and ask questions, etc. If there's any discovered bugs or issues, upgrade to the latest version is what you'll be told.
So yes in that you can keep getting training, calling tech support, etc. but nothing really "actionable" will take place, in my experience.
Also, there are worthy improvements made in future version, if you are paying for support, it's wise to upgrade, at least once every few years, etc.
The main reason to upgrade PDM other than support and new features would be to work with later versions of SOLIDWORKS files.
It sounds like they may be creating all their own data, and supporting outside file version, potentially newer than what they are using isn't an issue.
I have a few customers like this, they kinda upgrade when they feel like it....
Also to keep in mind your PDM needs to be at a version at or above your Solidworks. So no EPDM 2016 & Solidworks 2018.
As a reseller employee, I am happy to look at any software version that any of customers throw at me.Troubleshooting issues is the exact same process, no matter the version.
I will say if you find a 'bug' in an older version of software that cannot be reproduced in a current version of software, it won't be escalated up to development. It's essentially already fixed. In rare cases though, hot fixes can be requested but a business impact report must be submitted.
These are all great answers. I appreciate everyone's responses which will help me present the risk to our SOLIDWORKS sites.