Fun fact: the word "shard" in this context was coined by the dev team of the first major commercial MMO, Ultima Online, as an in-universe explanation for why the game world was split into multiple servers.
If you're looking for an exemplary server architecture for MMOs, look no further than UO. That game came out in 1997 and supported worlds with thousands of players and millions of persistent objects. Players could build their own houses within the game world, decorate (and later design) them to an incredible degree [1], and have other players visit (welcome or not). Players could run their own shops within their homes and sell their own hand-crafted, signed goods (weapons, armour, clothing, furniture) in addition to any found treasure they wanted to offer.
What made UO so impressive, technically, was how they accomplished all of this on such modest hardware as was available in 1995 [2]. This second link is to the GDC postmortem of the game. If you're interested, I recommend you check it out. It may not answer all your technical questions but it's extremely interesting nonetheless. The story of how they had to shard the game is part of the talk.
I'm pretty sure they did not use any off-the-shelf databases as the performance would have been terrible at the time. Everything would have been custom. The custom networking protocol is extremely reserved (not chatty), since it was designed for dial-up modems. Even when you run the game on modern hardware with broadband, the game uses on the order of a few megabytes per day of traffic.
I have heard Richard Garriott claim credit for coining "shard," and I'm very skeptical. The first database paper I could find that uses "shard" (as an acronym: System for Highly Available Replicated DAta) is from 1986: https://dl.acm.org/doi/book/10.5555/889956
Those authors may have been first, but I seriously doubt they were a significant influence on the term's popularity. If there's one thing I've learned from reading a lot of CS papers, it's that pretty much any word you pick out of a hat has been used as an acronym by somebody.
The paper you link to doesn't use "shard" as a noun, and in fact it doesn't describe a system that has "shards" in the sense that UO and just about everyone else since then has understood that term, i.e. independent partitions of a dataset:
> The reader is referred to [SBK] for a detailed description of the architecture of the SHARD system. Briefly, the main ideas are as follows. The network consists of a collection of nodes, each of which has a copy of the complete database.
(emphasis mine)
Additionally, according to Google Scholar, that paper was only ever cited 21 times. I checked the 16 of those citations for which full-text is available, and not a single one of them used the word "shard" as anything other than a passing reference to the name of the system itself.
If you're looking for an exemplary server architecture for MMOs, look no further than UO. That game came out in 1997 and supported worlds with thousands of players and millions of persistent objects. Players could build their own houses within the game world, decorate (and later design) them to an incredible degree [1], and have other players visit (welcome or not). Players could run their own shops within their homes and sell their own hand-crafted, signed goods (weapons, armour, clothing, furniture) in addition to any found treasure they wanted to offer.
What made UO so impressive, technically, was how they accomplished all of this on such modest hardware as was available in 1995 [2]. This second link is to the GDC postmortem of the game. If you're interested, I recommend you check it out. It may not answer all your technical questions but it's extremely interesting nonetheless. The story of how they had to shard the game is part of the talk.
I'm pretty sure they did not use any off-the-shelf databases as the performance would have been terrible at the time. Everything would have been custom. The custom networking protocol is extremely reserved (not chatty), since it was designed for dial-up modems. Even when you run the game on modern hardware with broadband, the game uses on the order of a few megabytes per day of traffic.
[1] https://duckduckgo.com/?q=uo+house+decoration&t=osx&iax=imag...
[2] https://www.youtube.com/watch?v=lnnsDi7Sxq0