I recently updated a 1.17.1 server (that was originally created on 1.16.3) to 21w39a. Aside from the expected extra lag and longer world generation time, the console gets filled with these errors:

[08:25:33] [Server thread/ERROR]: No key feature in MapLike[{blacklist:[],block_placer:

{type:"minecraft:simple_block_placer"}

,can_replace:0b,need_water:0b,project:1b,state_provider:{type:"minecraft:plain_flower_provider"},tries:64,whitelist:[],xspread:7,yspread:3,zspread:7}]; No key only_in_air in MapLike[{blacklist:[],block_placer:

{type:"minecraft:simple_block_placer"}

,can_replace:0b,need_water:0b,project:1b,state_provider:{type:"minecraft:plain_flower_provider"},tries:64,whitelist:[],xspread:7,yspread:3,zspread:7}]; No key disallowed_on in MapLike[{blacklist:[],block_placer:

{type:"minecraft:simple_block_placer"}

,can_replace:0b,need_water:0b,project:1b,state_provider:{type:"minecraft:plain_flower_provider"},tries:64,whitelist:[],xspread:7,yspread:3,zspread:7}]; No key allowed_on in MapLike[{blacklist:[],block_placer:

{type:"minecraft:simple_block_placer"}

,can_replace:0b,need_water:0b,project:1b,state_provider:{type:"minecraft:plain_flower_provider"},tries:64,whitelist:[],xspread:7,yspread:3,zspread:7}]
[08:25:33] [Server thread/ERROR]: Exception loading structure piece with id minecraft:jigsaw
java.lang.IllegalStateException: Invalid pool element found
at ddx.k(SourceFile:54) ~[server-21w39a.jar:?]
at java.util.Optional.orElseThrow(Optional.java:403) ~[?:?]
at ddx.<init>(SourceFile:54) ~[server-21w39a.jar:?]
at dem.a(SourceFile:74) ~[server-21w39a.jar:?]
at cwo.a(SourceFile:138) ~[server-21w39a.jar:?]
at cpq.a(SourceFile:429) ~[server-21w39a.jar:?]
at cpq.a(SourceFile:185) ~[server-21w39a.jar:?]
at abp.m(SourceFile:580) ~[server-21w39a.jar:?]
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1764) ~[?:?]
at asx.c(SourceFile:151) ~[server-21w39a.jar:?]
at aca$b.c(SourceFile:534) ~[server-21w39a.jar:?]
at asx.z(SourceFile:125) ~[server-21w39a.jar:?]
at aca$b.z(SourceFile:543) ~[server-21w39a.jar:?]
at aca.d(SourceFile:278) ~[server-21w39a.jar:?]
at net.minecraft.server.MinecraftServer.bf(SourceFile:770) ~[server-21w39a.jar:?]
at net.minecraft.server.MinecraftServer.z(SourceFile:758) ~[server-21w39a.jar:?]
at asx.bp(SourceFile:110) ~[server-21w39a.jar:?]
at net.minecraft.server.MinecraftServer.y(SourceFile:742) ~[server-21w39a.jar:?]
at net.minecraft.server.MinecraftServer.x(SourceFile:693) ~[server-21w39a.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:274) ~[server-21w39a.jar:?]
at java.lang.Thread.run(Thread.java:831) [?:?]

I only get these if I join the game around the oldest area on the server. I have also noticed that there doesn't seem to be a single item frame in the entire world, even though I swear I placed one before the switch. These errors do not crash the client or server.