Despite the description, the "We Can't Go Deeper" advancement actually does require the user to enter the nether.

To replicate, first build a nether portal and observe the advancement is not given when the portal is "lit" and the frame fills with netherrack.

To receive the advancement, one can teleport to the nether with the command /execute in minecraft:the_nether run tp @s ~ ~ ~

Modpack version:
1.0.4