Yeah these days xylazine is exceptionally common in stuff sold as "heroin" but it seems to be almost exclusively found in powders. Never seen an example of xylazine being present in tar, doesn't mean it doesn't happen.
But generally, while xylazine can cause some parathesia it probably isn't as intense as the reaction described here (and probably of a different nature). Who knows though.
On a side note, I remember being in a similar place where heroin no longer got me high.
To the OP, there is never any judgement on here, but get on methadone before you kill yourself will this shit! Methadone worked for me.
Then again, at the same, I was still shooting fentanyl for a while on top of my methadone. But i was using fentanyl patches.
I guess the problem with being on methadone maintenance these days is that the blocking effect isn't as relevant as it used to be. Back before fentanyl was commonly on the street, it was hard to find an opioid potent enough to allow a user to get high while on heavy dose methadone maintenance. I was on 380mg of methadone a day at one point, so the only way to get high was by shooting entire 100ug/hr fentanyl patches (almost 20mg of fentanyl). But they were hard to get and very expensive so that was somewhat self limiting.
But these days you can get fentanyl for dirt cheap, so it is now quite easy for someone to regularly get high while on methadone. I'd probably be dead if cheap fent was available 15 years ago.