I didn't ride a couple months because of foot surgery but have ridden a couple times about a week or two apart. No sense of loss of clutch those times or I did not notice.
The slave cylinder looked ok except for some discolorations, freckles. Look like metal coating imperfections not wear or o-ring material. O-ring looked fine through a magnifying glass. Probably the o-ring dried up too much while idle and fluid leaked into engine case then first use could have pinched or twisted the o-ring losing the rest of the fluid.
Ordered the Slave cylinder kit already, will get a countershaft o-ring or two for spares.
Get out and ride
2007 KTM 990 Adventure
2006 KTM 640 Adventure
2002 KTM 400/453 EXC