What is the Universal Rating System (URS)? While watching this year's 2020 Champions Showdown Chess 9LX I noticed they gave players' ratings according to the URS or Universal Rating System. Which are the characteristics of this rating system?
The Universal Rating System is an alternate rating system which, according to the project's web site:

The Universal Rating System™ (URS™ for short), is a revolutionary new
sport’s rating system designed to assess the relative strength of
participants across a wide variety of competitor vs competitor sports
or games.
The URS™ was developed as the result of a collaborative research
project funded by the Grand Chess Tour, the Kasparov Chess Foundation
and the Chess Club and Scholastic Center of Saint Louis.

According to the project's FAQ page, the major differences between URS ratings and FIDE Elo ratings are:

There are many differences between the URS™ and the FIDE system. The
most striking difference is that the URS™ calculates only one rating
for each player, informed by their results at all rates of play. This
published rating is the URS™ system's assessment of each player's
strength at Classical chess (defined as a rate of play where each
player has at least 2 hours for their first 60 moves). The URS™
recognizes that there is useful information about a player's
over-the-board chess strength in all game results regardless of the
time limit, and can therefore more effectively estimate a player's
Classical chess strength by also considering their results at games
played at faster time controls. As the speed of play increases, the
URS™ assigns less and less importance to the game results relative to
games played at slower time controls.
Furthermore, the URS™ is a weighted performance rating, calculated
across several years of previous game results for all players. Older
games are given less importance than more recent games, by applying an
exponential decay rate. Unlike traditional performance ratings, which
are based upon the opponents' rating at the time the game was played,
the URS™ constantly reassesses the strength of all opponents and
searches for the most likely set of current ratings for all players,
in order to best explain the observed results.