Modbus.org had provided free downloads of its serial and TCP protocol standards for years, until recently.

Access to the Modbus.org download page now requires registration - paid registration. An .edu student registration is reported to cost $500 USD.

There are other sites that have posted the standards, so if you need to reference the standard at some point in the future you might want to grab it now and save it.  The first three are the most widely referenced.

Modbus RTU
Modbus Over Serial Line Specification and Implementation Guide, V1.02
https://github.com/fawno/Modbus/blob/master/doc/Modbus_over_serial_line_V1_02.pdf

Modbus/TCP
MODBUS MESSAGING ON TCP/IP IMPLEMENTATION GUIDE, V1.0b
https://github.com/epics-modules/modbus/blob/master/docs/source/Modbus_Messaging_Implementation_Guide_V1_0b.pdf

All Modbus: Function Codes
MODBUS APPLICATION PROTOCOL SPECIFICATION, V1.1b
https://github.com/saisesai/modbus/blob/main/docs/Modbus_Application_Protocol_V1_1b.pdf

MODBUS/TCP Security, Protocol Specification-V21_2018-07-24
https://github.com/cazure/small_modbus/blob/master/docs/MB-TCP-Security-v21_2018-07-24.pdf

Conformance Test Specification for Modbus TCP Version 3.0 2009
https://assets.noviams.com/novi-file-uploads/modbus/pdfs-and-documents/MBConformanceTestSpec_v3_0.pdf