That's a good question...and perhaps I should research that. I have an 8-speed. (And by the way I love it...wouldn't "upgrade" it for anything). Haven't had any problems.
I think it depends on who you ask. Under Shimano's plan of forced technical obsolescence a lot of things were supposed to go away...but you can still buy 8 speed bar-cons from them. (An LBS told me they weren't made anymore. I told them to look it up. They were pleasantly surprised.)
The other question is what is the supposed "timeline" for this? Five years? (doubtful) Ten years? (Who cares at that point?)
Basically, I don't know if it's true or not, but I doubt you'll be looking for a part next year and find that they're all gone.
I hope they don't disappear...I really prefer 8 speed.
In fact, I changed my rear cassette not too long ago. Walked into the bike shop and said "I need an 8-speed 11-30." They said sure, here ya go. No problem finding replacement parts.
If 8 speeds were going to disappear very soon, I'd say you're right to be concerned. But I don't think it's soon at all. (I can still actually find 7 speed stuff!)