No, all SIP investments are not tax-free. Only SIPs in Equity Linked Savings Scheme (ELSS) can be claimed under section 80C of the Income Tax Act. While there is no maximum limit for investment in ELSS, the maximum tax rebate you can claim is ₹ 1,50,000 in a financial year.
Apart from ELSS funds, gains on all other SIPs are taxed as per asset class. For example, in case of equity-oriented funds, gains from sale of units before 1 year from date of purchase are taxed at 15% (Short Term Capital Gains) and 10% Long Term Capital Gains (LTCG) tax is applicable on gains if units are redeemed after 1 year (on gains exceeding ₹ 1 lakh in a financial year).