I have seen several reports saying that immigration now require a new TM 30 when extending a visa. Is this correct?

That entirely depends upon the office where you apply for the extension of stay (visas are not extended).

At some offices it would only be needed if you had never done one before or if you had left and re-entered the country since you previously did one.

Yes you now need TM30 to extend. I'm at immigration (Chaweng Watthana) now and had to join a separate 150 person queue to submit the TM30 form before they could process my visa.

 

The last two years I did not have to do this, so must have been added to the requirements in the last 12 months!