Re-add mention of discoraged proj keys

Heads-up @Nowosad

07-reproj.Rmd

@@ -115,8 +115,10 @@ They contain all relevant information about any given CRS, including its datum a
 Before the emergence of WKT CRS definitions, proj-string was the standard way to specify coordinate operations and store CRSs.
 These string representations, built on a key=value form (e.g, `+proj=longlat +datum=WGS84 +no_defs`), have already been, or should in the future be, superseded by WKT representations in most cases.
 
-Recent PROJ versions (6+) still allow use of proj-strings to define coordinate operations, but some proj-string keys are either no longer supported.
-<!-- `proj=`, `+nadgrids`, `+towgs84`, `+k`, `+init=epsg:` keys and only three datums (i.e., WGS84, NAD83, and NAD27) can be directly set in proj-string. -->
+Recent PROJ versions (6+) still allow use of proj-strings to define coordinate operations, but some proj-string keys (`+nadgrids`, `+towgs84`, `+k`, `+init=epsg:`) are either no longer supported or are discouraged.
+<!-- ref? (RL 2022-06) -->
+<!-- Second line was commented out (RL 2022-06) -->
+<!-- only three datums (i.e., WGS84, NAD83, and NAD27) can be directly set in proj-string. -->
 Longer explanations of the evolution of CRS definitions and the PROJ library can be found in @bivand_progress_2021, Chapter 2 of @pebesma_spatial_2022, and [blog post by Floris Vanderhaeghe](https://inbo.github.io/tutorials/tutorials/spatial_crs_coding/).
 As outlined in the [PROJ documentation](https://proj.org/development/reference/cpp/cpp_general.html) there are different versions of the WKT CRS format including WKT1 and two variants of WKT2, the latter of which (WKT2, 2018 specification) corresponds to the ISO 19111:2019 [@opengeospatialconsortium_wellknown_2019].
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