Initials of individuals in Messages

If you are referring to the left-most column listing the conversations and names, if that column is too small, it will revert to just initials instead of showing the names. If you carefully move your mouse over the boundary of that left-most column, the cursor will show arrows when it reaches the boundary. Then click and hold the mouse or trackpad to expand the width of that column. The names should return when it is made wider.

In Big Sur, you can visit Mail's View menu and deselect the menu item Show Contact Photo. That will hide the photos/initials column to the left of the Messages column.

In High Sierra, this is not on the Mail View menu, but in Mail Preferences > Viewing > Show contact photos in the message list.

It should not be grayed out. Not my experience here with Big Sur. Probably won't make any difference, but click on another message entry, and see if the menu entry changes.

You can force this feature off via the Terminal, provided your Mail preferences list is not damaged.

defaults read com.apple.mail EnableContactPhotos
defaults read com.apple.mail EnableContactPhotos -int 0

I can enable the Contact Photos from the Mail View menu, and enter the last command above to watch that initials column disappear from the open Mail interface.