Many players use autoexec configuration files to increase the number of FPS in their game. An outdated method constantly repeated in these old autoexec guides is to change your directx level by using the -dxlevel or -dxlvl launch option. This launch option changes the direct x level for the game and the changes are semi-permanent. The launch option is typically only added for the first launch and then removed for subsequent launches. The most commonly suggested direct x level instructed in such guides is -dxlevel 81, which would be Direct X version 8.1. Back in the day this made sense to do for older hardware, but today it makes no sense to utilize since the 2024 April 18 update (The 64-bit update) which enabled vulkan as the default render API. Forcing an ancient version of direct x will reduce FPS and introduce strange rendering issues, mainly in the backpack inventory and when rendering skins in-game. Direct X 8.1 was introduced in October of 2001 and was generally replaced by Direct X 9 in around 2006. If you are not using hardware from this era, there is no need to run a dxlevel this low. I hope this guide helps people with newer hardware or those who are just getting into autoexec's.

TL;DR - Since April of 2024 TF2 uses Vulkan as the default rendering API, and unless users are running GPU hardware from 2006 (or older) there is no reason to enable this setting.

For more clarification, here is an informative page straight from the Valve Developer Wiki:
https://developer.valvesoftware.com/wiki/DirectX_Versions