Good question.
I noticed recently that my number of solved problems had decreased by around 20 - 25, but could find no explanation.
Prompted by your question, I’ve just compared the list of my solved problems as shown by SPOJ, and the list from a folder where I’d saved the code, and the problems listed below are the ones I’ve solved that are now missing. Some of them were probably removed because they were far too easy, or poor quality, but surely not all of them?
- CHARGY
- CIJEVI
- COUNTDOWN
- CPRIME
- DIV02
- DP
- EQ
- ESY
- ETF
- GONDOR
- KRUS
- M00PAIR
- M3TILE
- MAJSTOR
- MCIRGAME
- MCONVERT
- MELE3
- MKUHAR
- MLASERP
- MMASS
- MMATRIX
- MMAXPER
- MMMGAME
- MRECAMAN
- MTOTALF
- MTWALK
- MZVRK
- P10878
- QUADRATE
- RELJEF
- ROADS
- SUM1
- TABLES
- WORDCNT
Edit: Some of them are still cached by Google if you search for the problem code.
Edit 2: Some more:
- BOXES
- FINFRAC
- HOUSES2
- MCLONUM
- MUL2COM
- TRAFFICN