A placeholder %discordsrvislinked_<placeholderapi_placeholder>%

That can parse directly from that player if he is linked for example:

Example 1

Bob (In discord): !playerinfo

DiscordSRVBot: parses the player name of the user cause he is linked. %islinked_player_name%
or maybe their group also %discordsrvislinked_luckperms_group%
etc.

Example 2

Ash (Unlinked user): !balance

DiscordSRVBot Sends %islinked_vault_balance%
it'll just send 'Unlinked user' cause he is unlinked.

Example 3

Gary (Linked user): !balance

DiscordSRVBot Sends %islinked_vault_balance%
it'll just send 'his balance from in game.

And this would be available in the DiscordCannedResponses etc, on top of this it'll give more customization overall.
And if the player isn't linked it'll just show 'Unlinked user' or something like that.

Especially on my server where it is required to link your discord account to join our discord server, it'd be really unique and fun.
and allow all the users to parse their placeholder information from discord.

So it's already possible to use PlaceholderAPI placeholders in DiscordCannedResponses expect for player related ones. What you want to do is using player related placeholders if the Discord user running a canned response has it's account linked.

Some things we have to check is what information about a player PlaceholderAPI's API requires from us. The problem is that the Discord user can be offline one the Minecraft server while using Discord and/or the account may be linked from another server in the same network and the user never entered the server this DiscordSRV instance is running on so there may be no information about the player except the UUID.
In summary: If PlaceholderAPI requires more then a UUID to process placeholders we can not implement this.