"Proxy" is a premium keyword in proxy business. Every proxy domain is unique and good for SEO to increase traffic. If you are interested in proxy business I´m sure you have tried searching if the word "Proxy" is available and found no TLDs available at the moment. But still many 3rd level domains available. Here is the list of all available proxy domains including 2nd level domains.

PROXY.COM.MU
PROXY.COM.AG
PROXY.NOM.ES
PROXY.COM.SG
PROXY.COM.SC
PROXY.ORG.MU
PROXY.NET.KI
PROXY.ORG.AG
PROXY.OR.AT
PROXY.ORG.SC
PROXY.FIRM.IN
PROXY.COM.KI
PROXY.CO.AT
PROXY.MU
PROXY.NET.VC
PROXY.NET.IN
PROXY.ORG.KI
PROXY.COM.VC
PROXY.NET.NF
PROXY.NET.MU
PROXY.ORG.VC
PROXY.KI
PROXY.NET.AG
PROXY.BIZ.PL
PROXY.TRAVEL
PROXY.AERO
PROXY.COM.NF
PROXY.NET.SC
PROXY.ORG.NZ