So, a big change in RBY history (thanks @Crystal_ and @Jørgen) showcasing how simulators have been wrong for years: Another unrelated mechanic being: rip RBY
Personally I think the way people have reacted by saying that Lax/Tauros will opt for different STABs is an overreaction, since BSlam is still awesome against a lot of things. While I don't think there'll be any moveset changes, it makes a huge difference to how the game is played, since Lax/Tauros can bash away at each other without getting crippled by status. Also unpara'd Chansey is even more of a nuisance now, thankfully it's probably the easiest thing in the game to land para on The other mechanic change you mentioned makes Slowbro a fair bit better but other than that isn't a big deal imo It's just amazing how we can still be discovering things about a game that's what, 20 years old now? Crazy (and cool)
Tauros' STAB is essentially set in stone, yes. But Snorlax does learn Mega Kick, which is 120 BP (albeit 75% accurate). With this development, it becomes a legitimate alternative that you'll see from time to time (although Body Slam's certainly still an option and probably still the standard). IE, that slot will probably end up as Body Slam/Mega Kick instead of just Body Slam 100%.
The speed-drop mechanic, in addition to helping out Slowbro, will also help out Tanklax, and could make Harden more appealing than Reflect (since Reflect doesn't call the code to lower the opponent's speed, and even if it did Reflect can only be used successfully once). If Snorlax and its normal-type opponent are both already paralyzed, Snorlax throwing down a Harden or Amnesia makes it faster. Even if the opposing Tauros takes you out first, you can bring your paralyzed Tauros in ... and the opposing Tauros is still slower! That's a bit more situational, but pretty huge.
Then again, Body Slam Tanklax is slightly weakened by not being able to paralyse Chansey on its own (only slightly weakened, because paralysing Chansey is pretty easy). And likewise I think this will make Withdraw (even more) preferred over Reflect on Tankbro.
The problem with mega kick is accuracy and pp, your oponent would keep recovering until you miss or lack of pp. After that, snorlax is nearly useless I hope they reset the server soon, cause I hate body slam paralysis >_<
Mega Kick does more damage than Body Slam even accounting for accuracy (120 * 3/4 = 90). Snorlax is now basically forced to Selfdestruct vs. unparalysed Chansey whether it uses Body Slam or Mega Kick (well, unless it can run Chansey out of Softboiled before getting killed), so there's no loss there. Non-Reflect Zam takes 69-82% from Mega Kick, so trying that is extremely risky (if it hits two MKs in a row, you're quite likely KOed even if you Recovered). Reflect Zam beats Snorlax anyway. Unparalysed Starmie is a problem, but remember that you're weighing that against Mega Kick saving you a slot for Hyper Beam; when you weigh Starmie's usage against that extra move (you're probably going to go EQ/Surf or Amnesia/Blizzard) it's not a terrible trade. Like I said, I don't think Mega Kick's going to take over 100% or even 50%, but it's now a legit option IMO. (And in all seriousness, in how many games is non-Rest Snorlax (the standard) actually going to use its STAB 9 times before Selfdestructing? It's bulky, but not THAT bulky.)
I'll have to look at it when I get home. Server was updated, so it should include this fix, but something might have gone wrong.