Well, from today, I discovered that I cannot use the old skype-126.96.36.199, where the SMS sending actually worked (that was apparently announced on End of Life of Skype for Linux 4.3); the Skype docs still refer to this old version:
Now Skype is only 64-bit, and has a deb repo: https://repo.skype.com/deb/pool/main/s/skypeforlinux/
... and it is now some Electron/Chromium application, and of course, there's no SMS - what I was otherwise paying for - anywhere:
On the left you don't have a contact list anymore, but "conversation" list, and if I do raise the contact list and right-click a contact, there's no "Send SMS" anywhere; on the chat box there's only "Open Moji picker" and "Add media or files", again no "Send SMS".
(Btw, they've done the same on Skype for Android - no SMS there either)
So, I thought, maybe I can use Skype for Web, but:
... "we're a bit overloaded".
So, how can I send SMS now?