To import QIF into Quicken (to import a QIF file into Quicken 2005-2014, 2015, 2016, 2017, 2018, 2019), you need to create a correct QIF file and follow the instructions below.
Quicken imports QIF and QFX (Web Connect) files with transactions. Once Quicken becomes three years old, it stops import QFX (Web Connect) files. Investing in a converter that creates QIF files from your CSV or OFX or QFX or QBO files is a good idea because you keep using your Quicken version until you see features worthy upgrade to new Quicken version.
There is also QXF files that you do not want. Unlike QFX (Web Connect) and QIF files, QXF replace whole data file and can create only from another Quicken that is the same year and the same platform (PC/Mac) as you have. QIF and QFX (web connect) files add transactions to your Quicken file, not replace them.
Yes, Quicken says QIF files are not supported, but yes, QIF files correctly prepared are importable directly into bank or credit account in Quicken. Import QIF into Quicken gives you ability to supply transactions, as QFX (Web Connect) format does not provide them.
Quicken 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2017, 2018, 2019 import QIF files fine for all account types. Your QIF file must be correctly created to import directly into an account you need it to imported, and CSV2QIF helps you create such QIF files.
When you convert a CSV file to QIF using CSV2QIF, enter the account name exactly as you have in Quicken and select the account type (Bank for checking and saving accounts, CCard for credit card accounts, Invst for investment accounts).
When you convert a bank file (OFX, QFX, QIF, QBO) to QIF using Bank2QIF (Bank to QIF Converter), load the file and click on the Account Mapping tab and enter the account name as you have in Quicken for each listed account.
Once you saved the QIF file, do the following:
Start Quicken and click File, then File Import, then QIF file.
On the import screen, select from the account drop-down list. Ignore the Quicken message about QIF limitations.
Review your imported transactions (Done!).
Quicken 2015, 2016 have the limitation with QIF import for bank and credit card accounts. You have to use a workaround with a proxy cash account, import transactions there, and then move transactions under a bank or credit account. There a separate tutorial for this conversion and import.