FabricTPA

A server-side Fabric/Quilt mod that adds /tpa command-set.
Works for Minecraft 1.16.2+ (snapshots not fully tested)
Requires FabricAPI

Commands

All commands except /tpaconfig don't require OP permissions. They're meant for everyone on a server to use.

/tpa <player> - Initiates request for you to teleport to <player>
/tpahere <player> - Initiates request for <player> to teleport to you

/tpacancel [<player>] - Cancel a tpa or tpahere request you've initiated, argument required if multiple ongoing
/tpaaccept [<player>] - Accept a tpa or tpahere request you've received, argument required if multiple ongoing
/tpadeny [<player>] - Deny a tpa or tpahere request you've received, argument required if multiple ongoing

/tpaconfig [<option> [<value>]] - Get/Set config options, more info below (Requres OP permission level 2)

LuckPers / fabric-permission-api

There are only 2 permissions:

fabrictpa.tpa allows players to execute the base commands
fabrictpa.config allows players to edit the config using the /tpaconfig command

Configuration

Configuration is done through /tpaconfig. There are currently 5 configurable options.
Configuration is also saved in config/FabricTPA.properties, from which the values are loaded at server startup. It also updates when a setting is changed in-game.

timeout - How long should it take for a tpa or tpahere request to time out, if not accepted/denied/cancelled. Default: 60 (seconds)
stand-still - How long should the player stand still for after accepting a tpa or tpahere request. Default: 5 (seconds)
disable-bossbar - Whether to disable the boss bar indication for standing still, if set to true will use action bar for time. Default: false
cooldown - The minimum time between teleporting and the next request. Default: 5 (seconds)
cooldown-mode - The mode for the cooldown, one of 3 values: WhoTeleported, WhoInitiated, BothUsers. Default: WhoTeleported. More info below

Cooldown modes

WhoTeleported - The cooldown is applied to whoever got teleported
WhoInitiated - Cooldown is applied to whoever initiated the request
BothUsers - The Cooldown is applied to both players involved in the teleport request