What is a “special tool”? A Philips screwdriver is pretty clearly not, but is a T-5 Torx? A security T-5? A Tri-wing? A Pentalobe?

"Special tool" is not used in the actual regulation; the requirement is that replacement must be possible with basic tools, defined:

> (50) 'basic tools' means a screwdriver for slotted heads, a screwdriver for cross recess screws, a screwdriver for hexalobular recess heads [Torx], a hexagon socket key, a combination wrench, combination pliers, combination pliers for wire stripping and terminal crimping, half round nose pliers, diagonal cutters, multigrip pliers, locking pliers, a prying lever, tweezers, magnifying glass, a spudger and a pick;

(Excepted devices can require "commercially available tools" which is defined exactly as you'd expect.)

https://eur-lex.europa.eu/eli/reg/2023/1670/oj

> A portable battery should be considered to be removable by the end-user when it can be removed with the use of commercially available tools and without requiring the use of specialised tools, unless they are provided free of charge […] to disassemble it.

> Commercially available tools are considered to be tools available on the market to all end-users without the need for them to provide evidence of any proprietary rights and that can be used with no restriction, except health and safety-related restrictions.

https://eur-lex.europa.eu/eli/reg/2023/1542/oj

Why does the Pentalobe exist in the first place?

Any tool you can’t get at a random local hardware store.