Canadian website:
https://pinksolution.ca/

I am sure they sell it in US too. Unfortunately for me they don't sell it in EU so I have to literally beg friends to get me a tub when they go to Canada.

This stuff works on everything AFAIK. I tried it on red wine stains, old coffee stains, grass, beet root,
curry, mud, ink, etc. Wish they'd start selling it here.

And the weird part, I can't figure out what is so special in mud spray coming up off a tire to make such a stubborn stain. In the fall, I blamed it on tannins fom our copious crop of fallen oak leaves, because the mud puddles looked like very strong tea. But now, just mud.

Weird!

Well, if nothing works, the only thing to do is to order a new shirt, and use the stained one as a "spare" after a soaking!

Glob it onto stains and gently press into fabric. Leave a pretty thick layer of it on stain. Check in about 30 minutes. If still there, leave on another 30 mins. Check every 30 mins until stain is gone.

If you are worried about color, do a tiny test patch on an inside seam first. (I haven't had problems with this mixture bleaching anything but just in case...)

When the stain is gone, toss it with the paste still on it into the machine and wash as normal. Vinegar smell will wash away, so don't worry about that.

Let you know how it goes.

Also, should clarify that when I say "thick" I don't mean so thick that it is crumbly. Needs to be "wet" enough to penetrate into the fibers somewhat.

It will bubble like crazy when you first combine the two, so use a bowl large enough to contain any initial exuberance of the mixture.