I've read about these in the manuals, but have not come across any in TS yet. Has anyone seen these ESI (Emergency Spreed restriction Indicator) signs in TS? Presumably they're only likely to be encountered in a scenario?
Yes, they are in TS, and need to be placed in a scenario. Can't remember which packs they come in though.
I've come across temporary speed restrictions in WLoS and WCML Trent Valley but don't recall if they used exactly these signs.
They are temporary, but are called ESRs because they are unplanned and not included in the driver’s WON (Weekly Operating Notice). It’s a warning sign for one. http://www.railsigns.uk/photos/p_tsr1/p_tsr1.html
From among the common items, London-Peterborough has them (the HST scenario). A German variant is used in the vR 01 pack, DB 1970s Freight (overnight track works scenario). Otherwise I'm in the same boat, I've seen them aplenty but couldn't list. A curious variant is used on Edinburgh-Glasgow, where they are placed in front of closed doors. The on-track ones.