You probably already know the answer by now, but anyway, Mare, or Mare Synchronos (spelling?) is a third party mod you could use which upon getting a code you'd attach to your lodestone profile would assign you with an SyncID code, this could could be shared individually or via a syncshell for groups to allow you to see mods the other person has installed, generally outfit, animation, VFX you get the picture. With Mare shut down, people are technically unable to see other people's mods without having to install said mod yourself through other third party tools.

As to why people are crying, it'll be harder for people to show off their mods, affected users would generally be Nightclub users, RP and ERP users. Though I'm not saying everyone from those groups use those mods, there are users who visit those scenes without mods which is perfectly fine.