Even the historical international brigades practiced some opsec and general common sense, you didn't just go and join. You were vetted by your local section of the Communist party, the international organized them and sent them out. Of course today's adhoc foreign legions just get instantly obliterated, there's no discipline and there's no wider movement helping to organize things it's just a sea of individuals trying to purge their own senses of alienation from being lotus-eaters in the treat-laden west. Even back in the 30s with an honest to god disciplined international organization with powerful nation state backing, things broke down.

Modern international brigades would need months of political education and cadre training just to get people on-board with the basics of internationalist socialism never mind weapons and physical training. Or we could hope the PSL (or equivalent) sponsoring you counts enough for political training. You'd have to cut people off from the internet and not let them use cellphones to call home at all - I honestly think modern undisciplined westerners of today would be unable to cope with what it would take. They'd only be allowed to contact home from the one phone or write a letter once a month.

Modern Ukraine's foreign legion was a meme they shrugged their shoulders at and said "yeah sure." They didn't organize it, the people coming in didn't organize anything, the nation states people were coming from disavowed it and didn't help organize it, they just showed up and some batshit midlevel Ukrainian officers with fantasies of Enemy At The Gate (who they wouldn't let near the regular army) were put in charge of this band of losers and turning them into a brigade. The YPG had a reason to accept international brigades based in theory and had a disciplined training program to onboard new fighters, and they didn't just take anyone with a pulse.