Midi-chlorians are fully canon. They exist in the Star Wars universe and have been mentioned in a wealth of other canon works including;
Star Wars: Revenge of the Sith
“According to the legend,” Palpatine said, “he could directly
influence the midi-chlorians to create life; with such knowledge, to
maintain life in someone already living would seem a small matter,
don’t you agree?”
Revenge of the Sith: Official Novelisation
Multiple episodes of Star Wars: The Clone Wars (S04E15 - Deception, S06E11 - Voices, S06E12 - Destiny)
I want to check his midi-chlorian count.
All energy from the Living Force... from all things that have ever lived, feeds into the Cosmic Force, binding everything and communicating to us through the midi-chlorians.
All that surrounds us is the foundation of life, the birthplace of
what your science calls midi-chlorians, the foundation of what
connects the Living Force and the Cosmic Force.
An episode of The Mandalorian (S02E04 - The Siege: Referred to as an 'M-Count')
Despite promising effects, the body rejected the blood. Pershing
doubts that they would find another donor with as high of an M-count,
referring to midi-chlorians.
Star Wars: The Mandalorian - Official Episode Guide
The Star Wars Book
A Force-sensitive individual is able to tap into the Force and learns
its will due to midi-chlorians: microscopic, intelligent life-forms
that communicate the will of the Force. They reside within the cells
of all living organisms and form a symbiotic relationship with their
hosts. Without midi-chlorians, life would not exist and the universe
would have no knowledge of the Force.
Star Wars: The Ultimate Visual Guide
Star Wars: Absolutely Everything You Need to Know, Updated and Expanded
Q: What are midi-chlorians?
A: Midi-chlorians are tiny, intelligent
life-forms that live inside the cells of all
living things in the galaxy. They do no
harm, and help their host use the Force.
The more midi-chlorians someone has, the
more powerful a Jedi they may become.
The High Republic: Trail of Shadows #1
Various articles in the Official Databank
But all traditions agree that life creates the Force and makes it
grow. Individuals’ Force potential depends on their levels of
midi-chlorians, microscopic life-forms found within living cells. The
Jedi believed midi-chlorians communicated the will of the Force, which
could be understood through meditation.
Star Wars: Databank - The Force
And used as recently as June 2022 in What a Jedi Makes, from the canon anthology Stories of Jedi and Sith
“Blood tests. They have my midi-chlorian count.” The boy pulled out a
flimsi from his rags. During his research, he’d discovered that the
Jedi often examined a candidate’s blood for microscopic organisms they
called midi-chlorians. The greater the number, the stronger a
candidate’s suitability to join the Order. Anticipating the Jedi would
request a count, the boy had paid an Ortolan bloodletter to perform a
test. He proudly pointed out the results to the girl. “As you can see,
my count is high.”