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PTC no longer supports any version of MathCAD or MathCAD Prime prior to 7 due to licensing agreements of components of code used in the program.
https://www.ptc.com/en/support/article/CS344202
It's funny to see that the support article does not mention Mathcad 14, 13, 12....
Luc
I'm pretty sure that's because you already couldn't download Mathcad 14 and older in 2021 anyway.
But people may still have CD's with accompanying license info of versions below 15 and would (even be in their right to) assume that that license is valid indefinitely.
Luc