Upon request, I prepared a guide to flag ESP files as ESL.

Why you would want to do this:
When flagged as ESL, the files do not count towards the 255 mod limit.

Why you might not want to do this:
However, only smaller mods can be changed to ESL. Furthermore, mods that are referenced by other mods and mods that are part of a save game should not be changed. (I didn't try, but I believe that this breaks the game.) Finally, if there are NPC records in the mod, this is going to create a Dark Face Bug. To fix this, you can use the Creation Kit and follow the instructions in the link.

Requirements:
1) SSEEdit
2) If there are NPC records, also the Creation Kit. (Install Bethesda.net Launcher from their web site, then you can install the Creation Kit)

Step 1: After installing SSEEdit, start it. It will go through all of your mods and load them.

Step 2: Find the mod. It will have a "mod id" between square brackets, based on its load order. For example, Skyrim will have `[00]`, and Dragonborn may have `[04]`. If this "mod id" starts with `FE` and is followed by three characters, the mod already has its ESL flag set and does not need to be converted. For example, I set the ESL flag for SV Beards and now it shows up as `[FE 015]` in my load order.

Step 3: Right-click on the file, and on the menu, choose "Compact FormIDs to ESL". This is going to renumber all of the records. If there are too many records, this will not work.

Step 4: Expand the file by clicking on the small plus sign to its left. Click on File Header right under the file name.

Step 5: On the right, you should be seeing the "Record Header" and "Record Flags" under it. If you do not, choose the "View" tab underneath.

Step 6: Double-click on the empty cell to the right of "Record Flags". You will see a list of flags, most of them are marked as "Unused", but one is marked as ESL. Check this box.

Step 7: Close SSEEdit and save the file. The file is overwritten.

Steo 8: If there are NPCs in the mod, open it in the Creation Kit, find the NPC and export the texture and mesh by hitting Ctrl + F4. This is explained in further detail here. If you get warnings, you can hit Yes or Yes to All. You can find out if there are NPCs in the mod when you expand the records with SSEEdit: if there is an entry called "Non-Player Character (Actor)", there are NPCs and their faces need to be exported using the Creation Kit.

Here is another guide if you want to learn more, it might be a bit outdated.

https://www.nexusmods.com/skyrimspecialedition/mods/21289?tab=description