Lower spec Arduinos only. There are plenty that are too fast. All Raspberry Pi's are banned, including the Pico.
10Mb/s networking is allowed in half duplex mode but full duplex hits the sanctioned speed. Dial-up and ISDN internet are in but broad band would require a wide interpretation of "up to" to be slow enough. (Starlink is way too fast)
Floppy disks are OK but all hard disks with an IDE interface or better are out. ST-506 and ST-412 hard disks are slow enough. Forget PCIe, PCI and ISA cards. CD-ROMs are theoretically OK up to x16 speed as are x1 speed DVDs but I do not remember any with a slow enough interface.
There is no need to go back to PS/2 keyboards. USB1 is slow enough even at full speed. Video is a problem: 640x480@60Hz black and white looks OK at first sight (18.432Mb/s) but the extra bandwidth required for overscan would take that over the 20Mb/s limit. Colour or grey scale really cut into the resolution or frame rate. ZX Spectrum graphics might just fit under the limit even when you include the "4bits/pixel" output rate.