Hot-swappable RAM was and is a standard feature on mainframe equipment (along with often hot-swappable CPUs), and was also a common feature on higher-end server

Hot-swappable RAM was and is a standard feature on mainframe equipment (along with often hot-swappable CPUs), and was also a common feature on higher-end server equipment through the '90s (for example Sun). It also appeared of course in some of the more in-between architectures such as HP Tandem/NonStop, which also featured hot-swappable CPUs. It was not unusual on these machines to have flashing status lights which would lead a technician to which drawer to open and which module in that drawer to swap out. There wasn't a particular niche for this equipment, it was just intended to be repairable while in service. At the time, it was typical to spend a larger dollar amount on equipment which was manufactured for very high serviceability. A combination of improvement in the actual reliability of hardware, increasing cost pressure, and probably as the lesser factor a shift towards more horizontal architectures (where redundancy was across machines instead of across components in machines) has made this a lot less common, although still not at all unheard of on high-end equipment.