It depends on both the level of the textbook and the markets it covers. If your book covers ordinary government and corporate bonds, I would suspect a book even 10 years old remains highly relevant. Likewise if the book focuses more on the math of fixed income than on the instruments themselves, as the math used today really hasn't changed much since the innovations of the 1980s and early 1990s. Many other very large markets (prime agency mortgages, agency bonds, emerging market bonds) also have not changed much. Even some smaller markets, such as credit card and auto loans, floating rate securities, and inflation-indexed products, haven't changed much.
On the other hand, a few fast-moving markets, such as structured and securitized products, have changed dramatically due to the global financial crisis of 2008. CDOs were a hot topic in 2005, but are virtually a dead market today. CMBS has transformed quite radically. CDS has a host of new conventions and rule changes.
Overall, though, if it's a good book, I wouldn't refrain from reading it out of a fear of learning about products that are no longer popular. If they are in the book, they were probably popular at one time, and it is worth understanding these products and why they failed (e.g., CDOs).