Right, the problems with measurements went out in the 1970s.
Unfortunately they dragged on a little longer. I have Mavic and TA parts from the late eighties (Mavic date coded most of their stuff) that are still French threaded. The mtb boom saw off most of the old French standards - and nearly killed the 650B tyre - but that wasn't until the late eighties.
I think the threads are more like obscure rather than obsolete. They still make spindles that fit.
I'm happy to call a standard that no major manufacturer has touched in twenty years obsolete. I can think of two French bottom brackets in current production, and they exist only to keep a few old gems running. Here in France a lot of frames get re-threaded. It's cheaper than a Phil Wood, and guarantees future availability of parts. I don't know of anyone making 25mm metric headsets.
That's not to say this stuff isn't available of course - eBay.fr is one source - and it's part of the game of playing with old bikes.
I've had problems removing and replacing pedals but I may be wrong. I thought the bottom bracket was the main problem.
The metric steerer and matching stem quill is often a bigger deal. The bottom bracket shell can be retapped if necessary. Metric cranks can be retapped to BSC.
But this is all just unwanted hassle. It would take a pretty special bike to make me want to dabble in French standards any more than I already have. That's not to say there aren't ways around most of the problems, but really, who needs it?
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.