Based on community discussions and GitHub archives, here are some clients known to include X-Ray features:
Method 1: Eaglercraft X-Ray Texture Packs (The Safest Route) xray hacks for eaglercraft
Warning: These can be unstable or potentially contain malicious scripts. Xray Hacks for Eaglercraft 1.5.2 vs 1.8.8 Based on community discussions and GitHub archives, here
The only reliable X-Ray for Eaglercraft is building a custom client with X-Ray built into the rendering engine. xray hacks for eaglercraft