Hi, so the serral vs neeb series is actually a really good example of top level zerg decision making and metagaming.
In all three games, serral goes lair before 3rd. In the first game, he scouts the sg so he knows it’s not a glaive all-in, making it safe for him to go 2 base hydra. The only protoss build that might kill him would be an oracle/chargelot all-in and those are very unpopular. Neeb also doesn’t really 2 base all-in pvz, he’s more like the standard archon drop/glaive adept/oracle opening player that goes into a 3 base immortal archon, and I think serral knew this which is why he went for an extremely early hydra attack as neeb was setting up his third.
The second game lair vs sg again, but then serral decides to go roach sh nydus vs double sg, which is free win for protoss cause they just lift the hosts/queens and theres no more aa. I think serral was trying to go for a timing here that may have worked in practice, but it failed because sh need protection vs air and there will be more phoenixes than queens due to lair morph time, chrono, and only being on 2 hatch.
The third game was the same as the first for early game, except for the muta switch. What I think happened was that neeb prioritized storm and fourth base because he thinks he needs it to beat zerg/serral in a macro game, but serral knows this is what neeb will try to do so he just plays defensive with hydra ling bane and does a surprise switch into muta.
Also yes, in ZvT 2 base muta has become a lot more popular as 2-1-1 becomes rarer. The reason for this is that it’s much easier to defend vs hellion openers on 2 base, especially when you can afford an earlier wall off. Additionally, it will clean up banshees, liberators, or bcs, can chase hellions away as you establish third, and force earlyish turrets from terran, delaying his fourth/additional barracks. People say it loses to 2-1-1, but honestly you can afford baneling nest with this build and can delay spire for centrifugal hooks, but what is most important is to take a third earlier, like as soon as you can after you identify 2-1-1 because they can’t pressure you until 5 minutes and you will need the extra larvae from the third hatch.
That is basically it. If you see zergs opening earlier tech, it is because other races have become better at punishing such eco play, so zerg has to metagame and take control of the game with something like early mutas.
As an aside, zergs going mutas have historically been done out of necessity, and usually results in zerg eventually getting buffs when muta play gets figured out. We saw this in early wol, where zergs were going muta ling bane before the infestor got buffed, we saw this in HoTS TvZ, which was consistently terran favoured at the top levels due to non-stop inefficient trades with muta ling bane vs mmmine in the midgame, which zerg was forced into to deal with boostivacs and basically killed them in a war of attrition, and in the next expansion zerg got huge buffs many of which were specfically aimed at helping zerg vs this style. We are seeing mutas again increase in popularity after the nerfs in 2019, and what do you know, zerg winrates are falling.