The Venerable old PCI slot which first was introduced with the Pentium 60 MHz and 66 Mhz processors. PCI was intended to deal with the ISA slot limitations and the VESA local bus extension. There is no shortage of socket AM4 motherboards with available PCI slots. All of the AM4 boards in the studio are PCIe exclusively.
Today every function imaginable has long ago migrated to PCI Express. USB 3.0 is backwards compatible so existing USB devices are fine. Adapters for USB-C are widely available. USB cards of all kinds are inexpensive and are convenient to support front panel requirements.
Firewire PCIe cards are convenient for digital audio and video needs. 16 channel mixing boards with Firewire can be fed into a sound mixing tool in Windows. More recent hardware has adopted USB.
SCSI cards that support even the old 50-pin cable are available for legacy device support. One card was seen with full support for several SCSI cabling needs. The competition from SATA 4-port cards changed disk interface with a standard everyone could use.
PCIe cards with serial and parallel ports are low cost. So it means headers on a motherboard can swap out for USB headers and RGB fan controls.