Microsoft has changed several preset shortcuts as discussed in the article at https://techcommunity.microsoft.com/t5/microsoft-365-insider-blog/new-zoom-shortcuts-in-word/ba-p/4219049.
According to the article, Subscript is now Ctrl + Shift + <hyphen>.
You can set your preferred shortcut via the Customize Keyboard dialog box. Click File > Options > Customize Ribbon > Customize to display the dialog box. Choose the "All Commands" category and find the Subscript command. Click in the "press new shortcut key" box and enter your preferred shortcut. Make sure that "Save changes in" is set to "Normal" and click the Assign button.