In practical terms, the test applied is probably more one of "does not continue to exercise the rights of foreign citizenship" rather than "renunciation of foreign citizenship".

If you continue to hold a passport from a foreign country at the age of 18 years 6 months, or reside in a foreign country as a non-alien, or vote in foreign elections, etc... then it can be reasonably argued that one has not renounced foreign citizenship. The burden will be on the Government of India to demonstrate that one has NOT renounced foreign citizenship if one were to make a declaration that one has indeed done so.

In the case of a relative of mine who was born outside India to Indian citizen parents and continued to reside outside India till he reached the age of majority, the Indian High Commission required him to sign an affadavit renouncing Indian citizenship by descent before they would issue him a tourist visa on his foreign passport after reaching the age of majority. He can however reclaim Indian citizenship by registration at any time he chooses, but will need to renounce his other citizenship in order to legally do so.

The law really isn't that complex if you consider its underlying concept - viz. one can only hold allegiance to a single master at any time. All the laws written around it simply seek to uphold this principle. The only people who complain about it are those who are unwilling to accept the concept above.