This is my favorite spreadsheet: https://rpggeek.com/filepage/108409/bestiary

It lists every published monster to date, has around 20 columns of information about each one and has wonderful sorting features built in. It is updated within a week or two of each book being published.

Using an online Challenge calculator, I am not seeing the issue that you are describing.

Monsters exist up to CR 24, and then the Tarrasque by itself is 30.

For 4 Level 20 PCs, a single enemy of CR 23 is at the high end of a Hard encounter, just barely below the threshold for a Deadly encounter. A CR 24 is thus a Deadly encounter, but 5 Level 20 PCs push it back down to a Hard. (The Tarrasque requires 7 Level 20 PCs to push it under the Deadly level, but lets ignore that one! Using that, you are definitely threatening a TPK!)

I believe you are calculating challenge improperly. I will explain what I think you are seeing, and you can obviously correct me if I am off base.

For the CR calculations(using the table on page 82 of the DMG or page 56 of the Basic DM Rules), a level 20 PC has a Deadly threshold of 12700 XP. A single CR 15 monster is worth 13000 XP. Since that is higher, you see it as being a Deadly encounter. However, for encounter calculations, you must add in the XP threshold for each party member to arrive at your overall thresholds (which is explained in step 2 of the Evaluating Encounter Difficulty section on the same page as the XP Chart). For a party of 4 Level 20 PCs, the Deadly level is set at 50800 XP. A single CR 15 monster against a party of 4 Level 20 PCs will be an Easy encounter.