There is a bit of a misconception here regarding obsolete vs a tech trained, more accurately not trained, to work on a particular unit. When a new person goes through a tech course with SP they learn how to service the regs that are in the latest catalog, meaning only the current regs that are being sold.
It isn’t until a person goes through the third level (although that has changed recently) that a tech is introduced to the older regs. Some stages aren’t even covered in the course, such as the older D-series or early Air2 generations. Simply put, if a tech hasn’t been trained to work on a piece of gear then he/she should not work on it.
People can gain experience on an older unit by working with an experienced technician to learn how to properly service it, Another way a tech can learn about the older units it to obtain several older ones and work/play with them then get with someone who has knowledge and then take the level three course, back when it was offered.
For an individual who has a good foundation servicing regs can learn how to work on the older regs fairly easy. With all that being stated there are some units that SP no longer has parts for, but has the service kits. If a component is broken and not part of the service kit (the poppet spring and purge cover for the D300/350/400 second stage for example) then the tech is left with one of two options, obtain a part from somewhere, if possible or simply inform the customer he/she will be unable to service it. There are some stages that SP no longer has service kits for such as the Mk7 and 2nd gen Air2.
The G-series second stages from SP can be serviced if the technician has some sort of training via SP’s more advanced course or working with an experienced technician to learn more about older regs. If one observes the scuba industry you’ll notice it is based on an apprentice type training to build on what was learned in a course.
When it comes to an LDS/service center/resort, it must determine if it is worth getting the training and experience to service older units. To use myself as an example, it was worth it because I enjoyed learning about various regs and I ended up servicing a lot of older units that were still in decent shape. Sometimes, it wasn’t worth it.
Good luck.