Actually, it was a period when it did exist. Late medieval/renaissance. The rigidity was forming. Precedents counted. New precedents could be made... but could fall through.
Nope.
Making knights (of otherwise unobjectionable characters) was a common thing traditionally accepted for mid-level nobles.
Demoting knights while alive was unusual, and it was not going to be taken seriously by others unless the side purporting to demote was a recognized authority, like king. (Maybe popes or major republics could, not sure about their practice).
No. But reversing a demotion is far easier than making someone a virgin!
Counting a set easier to check: the English Knights of Garter who were degraded:
- Jasper Tudor - Lancastrian. Elected 1459. After his side´s loss 1461, fled abroad. Briefly returned with Lancastrian side 1470-1471, permanently 1485. Previous attainders annulled, thus restored to Garter.
- Thomas Grey - opponent of Richard III. Appointed 1476. Fled abroad 1483-1484, degraded. Also returned and restored 1485 with Bosworth
- Lovell "our dog". Appointed 1483. After Bosworth, fled abroad and was degraded. Missing, presumed dead after 1487.
- 1st Earl of Surrey. Appointed 1483. Captured alive at Bosworth, thrown to Tower, lands, titles and Garter knighthood taken away. After 4 years in Tower, pardoned, released, lands, titles and Garter returned.
- 3rd duke of Suffolk. Appointed 1499, fled abroad 1501, declared attainted 1504
- 3rd Duke of Buckingham. Appointed 1499. Arrested in April 1521, beheaded in May - attainder and degradation definitely posthumous
- 1st baron Darcy. Appointed 1509. Captured for complicity in a revolt February 1537, beheaded June, degraded posthumously
- 1st marquess of Exeter. Appointed 1521. Arrested November 1538 on accusation of treason, beheaded December, degraded posthumously
- 1st Earl of Essex. Appointed 1537. Arrested June 1540 on accusation of treason, and in his case degraded in lifetime. Beheaded in July
- Earl of Surrey, son of the next. Appointed 1541. Arrested with father in December 1546, beheaded in January, degraded posthumously.
- 3rd Duke of Norfolk. Appointed 1510. Arrested December 1546 on accusation of treason, sentenced to death, reprieved but deprived of lands, titles and knighthood, spent 6 years in Tower, released and lands, titles and knighthood returned on accession of Broody Mary
- 1st baron Paget. Appointed 1547. Thrown to Tower in 1551 as accomplice of Duke of Somerset, degraded and fined in 1552 - got titles back from Broody Mary.
- 1st Duke of Northumberland. Appointed 1543. Tried making Jane Queen, captured in July 1553, beheaded August, degraded apparently posthumously
- 1st marquess of Northampton. Appointed 1543. Sentenced to death with 1st duke of Northumberland (above), and titles taken away, but reprieved. After a few months in Tower released but without titles. Got titles back from Bess I in 1559
- Duke of Suffolk. Appointed 1547. Beheaded and degraded in 1554 for Wyatt´s revolt
- 7th earl of Northumberland. Appointed 1563. Fled abroad 1569, degraded, extradited and beheaded 1572
- 4th Duke of Norfolk. Appointed 1559. Arrested September 1571 for a treason plot, beheaded June 1572, degraded at some point
- 2nd earl of Essex. Appointed 1588. Captured in revolt 1601, beheaded 17 days after capture, degraded at some point.
- 11th baron Cobham. Appointed 1599. Arrested for treason plot 1603, degraded, imprisoned in Tower for 15 years, released when old and ill and died soon after
- 1st duke of Monmouth. Appointed 1663. Captured in revolt 1685, beheaded 7 days later, attainted in lifetime
- 2nd duke of Ormonde. Appointed 1688. Fled abroad 1715, degraded 1716
No Englishmen before or since - not counting alien enemies, who several times returned Garters of their own initiative.
So when you have a political revolution, it is not unlikely that some knights would flee abroad and be degraded in absentia - you have a list of 6 knights who fled abroad and were degraded in absentia. Degradation in absentia was not sure to stick - 2 of the fugitives managed to return (both with victorious Henry VII). When knights are thrown to Tower, they might be soon executed, or released without formal dishonour, but formal degradation while leaving the knight alive in Tower was not unknown - 5 ex-KGs lived to get out of Tower. One was restored by the monarch who degraded him (Surrey, after 4 years captivity), 3 by next rulers soon after accession (Mary restored 2 ex-knights, Bess I one), one was released but without titles or wealth.