Yes, the Horimiya manga has an epilogue. It was released in the 6th and final volume, issued in April 2020. The epilogue is set two years after the main story, and follows Miyamura and Hori as they enter into their third year of high school. It’s a touching moment for both of them, as for the first time in yesrs, Miyamura is finally free from all of his troubles and is looking towards the future with eyes of hope and optimism.
Many of the other characters also have their own epilogue as well. Yoshikawa, Sengoku, and Yuki all have sections of the manga dedicated to them and their own stories. In the special chapter of the Hiromiya fanbook, there is also an extra epilogue which tells a side-story from the morning after Hori and Miyamura have started living together.
The epilogue is a satisfying way to conclude the Horimiya series, and fans definitely should give it a read!