What the file is
server.properties sits next to the server's jar and is read once when the server starts, so restart the server after changing it. Every key here is one the server writes into a new file, with the default it writes. Checked against Java Edition 26.3; the list changes now and then, and the notes say when a key arrived or a default moved, such as white-list, which is on by default from 26.3.
Changed keys or every key
A server fills in any key the file leaves out with its default and then writes the whole file back. So a file with only your changed keys works as a starting file for a new server, and is easy to read. To replace the file on a server that has run before, paste the old one in first, so its settings are kept, or download every key.
Pasting your own file
Paste the whole file into the box and press Load it. The page reads it the way Java does, comments and escapes included, sets every key it knows, and keeps any key it does not know, such as one a mod adds, at the end of the file. Keys that have become game rules, like pvp and spawn-monsters, are pointed out: the server ignores them now, so set them with /gamerule in the world instead.
How values are written
The file is written as Java's Properties format expects: a colon, equals sign or hash in a value gets a backslash, which is why the server writes level-type=minecraft\:normal, and anything outside plain ASCII, such as the § in a coloured MOTD, is written as \u00A7 so it reads the same whatever the file's encoding.
Warnings
The page flags settings that let someone take over or slow the server: online-mode=false, which lets anyone join under any name, an operator's included, unless a proxy checks accounts; RCON with no password or a short or common one, since RCON gives full console control and sends its password unencrypted; and view or simulation distances past 12, which cost far more memory and processor. Bedrock Dedicated Server uses its own, different server.properties; this page is for Java Edition servers, including Paper, Purpur, Fabric and Forge, which all read the same file.
Questions
Why are pvp and enable-command-block missing?
Since 1.21.9 they are game rules, not server.properties keys: pvp, spawn_monsters, command_blocks_work and allow_entering_nether_using_portals in /gamerule, as named from 1.21.11. A server ignores the old keys if they are still in the file.
Can my players not join after updating to 26.3?
If the server made a fresh server.properties, white-list is now true by default, so only whitelisted players can join. Add them with /whitelist add, or set white-list to false. An existing file keeps the value it had.
Where do Paper and Spigot settings go?
In their own files: bukkit.yml, spigot.yml and Paper's config folder. Paper and Spigot still read server.properties for everything listed here.
Is my file uploaded?
No. Pasting, editing and downloading all happen in this page, on your device. Changed settings are kept in the page's address so a link brings them back, except passwords and secrets, which never go in it.